1. Field of the invention
The present invention relates to mezzanine cards such as those which may be used on VMEbus, CompactPCI, and other host processor boards, and more particularly to a PMC card which complies with the IEEE 1386 specification and permits the use of standard Type I and Type II PCMCIA cards.
2. Description of the Prior Art
Over the past few years, several backplane buses have evolved which permit the configuration of a computer system using standardized host processor cards which communicate along the backplane bus. Examples of such buses are the VMEbus and the CompactPCI. Generally, the host cards contain appropriate micro-processors, co-processors, and necessary associated memory. Many times the host processor cards do not provide for I/O. The host cards are constructed so that they can perform a general computational function and are further specialized by the individual auxiliary cards which mount onto the host cards. Since these auxiliary cards mount just above and parallel to the host card, the auxiliary cards are generally referred to as "mezzanine cards".
Typically, mezzanine cards have dimensions smaller than the host processor card and plug directly into the host card through appropriate connectors. Host cards often provide for more than one mezzanine card. A myriad of mezzanine cards have been developed for the board/backplane market. One source has identified at least thirty different mezzanine board variations just for use on the VMEbus. In order to eliminate the confusion and duplication required by having proprietary mezzanine cards for each of the host processor cards and each of the buses, the IEEE has been working to develop a single standard for mezzanine cards which would enable them to be used with any host processor on any bus.
The resulting IEEE 1386 specifications have two major components, a local bus requirement as expressed in IEEE 1386.1 and a mechanical requirement as Expressed in IEEE 1386. First, IEEE 1386.1 requires that the bus between the mezzanine card and the host card conform to the Peripheral Component Interconnect (PCI) bus. The Peripheral Component Interconnect bus is a non-terminated, non-incident wave switching bus running synchronously at 33 MHz, in typical implementations. It is intended to be a local bus as opposed to a backplane bus. PCI offers a parity-protected 32-bit multiplexed data/address bus, with an option for a 64-bit extension. Most PCI implementations use a 32-bit bus. Because of PCI's local-bus bandwidth--132 MB/s peak--high speed applications, such as 100-Base-T, Fiber Channel, and Asynchronous Transfer Mode (ATM) will work exceptionally well, as compared to implementations with prior-generation mezzanine card standards.
Using a PCI bus increases the transfer rate by as much as ten-fold over several prior generation proprietary mezzanine cards. At the same time, low speed applications, using only a small percentage of the available PCI bandwidth, also work well and provide cost-effective solutions. Finally, the PCI bus technology was chosen since it is found in tens of millions of desktop personal computers where the economies of scale have reduced the cost of PCI interfaces by taking advantage of the personal computer market's economies of scale. Mezzanine cards built to the IEEE 1386.1 standard with PCI buses are therefore known as PCI Mezzanine Cards, abbreviated as PMC.
Presently, one of the principal product lines available in the computer market are PCMCIA cards which are particularly prevalent in the laptop and portable personal computing market. PCMCIA cards are approximately credit-card sized modules which may contain any number of different functions. Typical functions available through PCMCIA technology are modems, flash mass storage, rotational magnetic storage, and LAN interfaces. Additionally, data acquisition cards are becoming available at reasonable cost. Ideally, it would be desirable to use the relatively inexpensive and widely available PCMCIA cards in I/O applications for the backplane market.
IEEE 1386 specifies a defined mechanical configuration for PMC cards. As can be seen in FIG. 1, a typical PMC consists of a front-panel bezel 1, bezel supports 2, stand-offs 3, printed circuit board 4, and at least two 64-pin connectors 5 towards the rear of the card. The front panel bezel 1 engages with the front panel of a host processor board producing an EMI-tight seal and providing a suitable opening for external I/O connectors. Typically, there are also threaded holes 6 provided in the bezel mounting supports 2 which permit mechanical attachment to the host board. The PMC cards are mounted parallel to the host processor card and are held in position by screws mounted through the host processor card into the stand-offs 3 and into the threaded holes 6 in the bezel supports 2. The rear 64-pin connectors 5 carry the PCI bus and user-defined I/O. In the terminology of the IEEE 1386 specification, the side 7 of the printed circuit board 4 facing the host processor board is designated as side 1 and the other side 8 of the printed circuit board 4 is designated as side 2. The IEEE 1386 specification provides for both a single wide as well a double wide PMC card. The single wide card measures 149.times.74 mm (roughly 6.times.3").
The "stacking height" of the PMC is defined as the distance between the Side 1 surface of the PMC printed circuit board and the surface of the host card printed circuit board immediately opposite side 1 of the PMC. The stacking height varies depending upon the type of the host processor board. For VMEbus host boards, for example, the stacking height is 10 mm. Other host processor boards may use a different stacking height. The stacking height is important for two reasons: (1) some space must be left for air flow between the PMC card and the host processor board; and (2) components on the host processor and the PMC must not mechanically engage/interfere with each other. As will be seen below, the IEEE 1386 specification for PMC cards standardizes the cards so that stacking height variations are governed solely by the mechanics of the host processor. In this way, standardized PMC cards can be universally applied in any stacking height context.
The component envelope for PMC cards includes the height of components on both sides of the board, plus the printed circuit board thickness. The component envelope is specified to be no greater than 8.2 mm in the primary component area of the board. Maximum height of components on side 1 of the PMC (which faces the host card) is set at 4.7 mm which is ample enough to handle most surface mount integrated circuit packaging. The maximum height (measured from the side 1 surface) of components on side 2 of the PMC is 3.5 mm. Considering that a typical printed circuit board is approximately 1.5 mm thick, approximately 2.0 mm remain for side 2 components. This height can accommodate SOICs, QSOPs, TQFP, passive components and other low-profile surface mount technologies. Since the PMC component envelope will always be no thicker than 8.2 mm, host card manufacturers are responsible for designing cards so that none of the host card components will mechanically contact a mounted PMC. For VMEbus host cards, a maximum height of components on the side of the host card facing the PMC card is set at 4.7 mm. Clearly, for other host processor cards with a different stacking height, components on the host card may protrude a greater distance from the host card without striking the PMC card. In addition, the stacking height for each host card implementation will govern how high off the host board the 64-pin connectors need to be mounted.
As noted above, in the ideal world it would be possible to use the mass-marketed PCMCIA cards with PMC cards. However, PCMCIA connectors, which provide both electrical and physical support, designed to receive Type I and Type II PCMCIA cards typically measure approximately 7 mm thick. This thickness is required to accommodate the 5 mm thickness of the Type II card as well as the 3 mm thickness of the Type I card. Some PMC cards capable of receiving a PCMCIA card are presently available in the marketplace. However, the manufacturers have mounted the PCMCIA connector on the surface of the PMC printed circuit board, and as a consequence have grossly violated the IEEE 1386 specification. The advantage of the standardization provided by the IEEE 1386 specification is therefore lost and these PCMCIA to PMC adapter cards must be checked against the configuration of each host processor board to see if there is mechanical interference. As the PCMCIA cards have become more popular and less expensive, there has been a strong desire to utilize these cards in conjunction with PMC cards, but until this invention, there has been no way to obtain the convenience of using a PCMCIA card on a PMC and retain all advantages of the IEEE 1386 specifications.